Method for preparing ZnO-TiO{HD 2 {B bichargeable electrophotographic material

ABSTRACT

An electrophotographic photosensitive material comprising a base and a photosensitive layer consisting mainly of photoconductive fine powders and an insulating resin binder, the photoconductive fine powders being a powder mixture of zinc oxide and an oxide of titanium obtained by the hydrolysis of an organic metal compound of titanium in a Zn:Ti mol ratio of 3:1 to 70:1, preferably 15:1 to 40:1, the mixed powders having been heat-treated at a temperature of at least 300*C, preferably between 500*C and 700*C is disclosed. This photographic layer can be charged both positively and negatively as desired.

1 1 Feb. 4, 1975 METHOD FOR PREPARING ZnO-TiO2 BICHARGEABLE ELECTROPHOTOGRAPHIC MATERIAL [75] Inventor: Hajime Miyatuka, Saitama, Japan [73] Assignee: Fuji Photo Film C0., Ltd,

Nakanuma Minami Ashigara-shi Kanagawa, Japan [22] Filed: Aug. 8, 1972 21 Appl. No.: 279,136

[30] Foreign Application Priority Data Aug. 12, 1971 Japan 4661142 [52] U.S. Cl 96/1.8, 96/1.5, 252/501 [51] Int. Cl G'03g 5/08 [58] Field of Search ..96/1.8, 1.5; 252/501 [56] References Cited UNITED STATES PATENTS 3,174,856 3/1965 Horne et a1 252/501 X 3,220,830 11/1965 Kashiwabara... 96/18 X 3,245,784 4/1966 Stricklin 96/1 3,429,662 2/1969 Klein et a1. 96/1.8 X

3,471,288 10/1969 Berman 96/1 3,630,733 12/1971 Manhardt 96/1.8X 3,630,743 12/1971 Harvill 96/].8 X 3,653,895 4/1972 Brandon 96/1.8 X 3,655,376 4/1972 Wood et al. .96/1.8 X 3,674,476 7/1972 Tamai et al 96/1 .8 3,684,507 8/1972 Francis 96/15 X 3,698,894 10/1972 Foss 96/18 X 3,717,462 2/1973 Negishi et a1. 252/501 X 3,754,906 8/1973 Jones et a1. 1. 96/18 X FOREIGN PATENTS OR APPLICATIONS 908,779 10/1962 Great Britain 96/18 253,153 3/1961 Australia 96/18 941,702 11/1963 Great Britain.... 96/l.8

6,812,232 3/1969 Netherlands 96/].8 46-21999 6/1971 Japan 96/18 OTHER PUBLICATIONS Keck, Photoconductivity in Vacuum Coated Selenium Film," Journal of the Optical Society of America, Vol.42, No. 4, April 1952, pp. 221-225. Zinc Oxide Rediscovered, 1963, pp. 6071.

Primary Examiner-Norman G. Torchin Assistant Examiner-John R. Miller Attorney, Agent, or Firm-Sughrue, Rothwell, Mion, Zinn & Macpeak [57] ABSTRACT An electrophotographic photosensitive material com- 10 Claims, No Drawings METHOD FOR PREPARING ZnO-TiOz BICHARGEABLE ELECTROPHOTOGRAPHIC MATERIAL BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an electrophotographic material, and more specifically, to an electrophotographic material which contains improved photoconductive fine powders and which can be charged both positively and negatively.

2. Description of the Prior Art Photographic materials widely used in typical electrophotography which involves the successive steps of uniform electrostatic charging on the electrophotographic photosensitive layer, image-wise exposure thereon and developing the thus obtained electrostatic latent image with toner are produced by forming on an electrically conductive base material a photosensitive layer comprising photoconductive fine powders of zinc oxide uniformly dispersed in an insulating resin binder.

A negative charge can be easily retained on a photosensitive layer comprising zinc oxide, but difficulty is encountered in retaining a positive charge. Therefore, the photosensitive layer comprising zinc oxide is charged only with a negative polarity, and when it is desired to change an original from a positive to a negative, the polarity of the developer must also be changed. In other words, the development ofa positive image is performed by a method (regular development) which relies on the electrophoresis ofa toner to an area in which an electrostatic charge remains, and the development of a negative is effected by a method (reversal development) which comprises applying a toner having the same polarity as the charge of the electrostatic latent image to areas other than the area in which an electrostatic charge remains. For this reason, since the polarity of the electrostatic charge on the photosensitive layer is negative, it is necessary to use a developer containing a toner of a positive polarity for the development (regular development) ofa positive image, and a developer containing a toner ofa negative polarity for the development (reversal development) of a negative. To replace developers containers toners of an opposite polarity to each other in the developing section of the equipment not only requires a complicated operation, but also leads to the likelihood that toners of different polarities will be mingled in the developer. This causes a decrease in the quality of the developed image. Furthermore, the reversal development of a negatively charged photosensitive layer requires the use of a developer containing a toner of negative polarity.

On the other hand, it is quite difficult to obtain developers with toners having a negative polarity in a stable dispersion. The major reason is that many of the color ing pigments, for example, carbon black or the phthalocyanine pigments, which constitute the toner have a positive polarity in an insulating liquid. Furthermore, coloring pigments are dispersed in a resin vehicle in the production of a developer, but since many of the vehicle resins (such as a vinyl chloride resin or nitrocellulose) for giving toners of a negative polarity have poor wetting properties to the pigments, it is especially difficult to prepare a liquid developer in the form of a dispersion with high stability. In addition, since many of the vehicle resins-are extremely difficult to dissolve, insulating liquids having considerably strong dissolving powers need to be used as the carrier liquids for the liquid developer, and the resin binder for the photosensi- V tive layer should have a resistance to such insulating liquids having a strong dissolving power.

On the contrary, it is far easier to obtain developers with toners of a positive polarity. For example, a number of resins such as alkyd resins or rosin-modified formaldehyde resins can be used as the resin vehicle. These resins have good wetting properties to the pigments, and it is not so difficult to select readily soluble resins.

It is preferred therefore that a developer containing a toner of a positive polarity be used, whereby regular development is performed by negatively charging the photosensitive layer when the original is a positive, and reversal development is performed by positively chharging the photosensitive layer when the original is a negative.

For the above reason, a photosensitive layer capable of being charged sufficiently both positively and negatively has been desired. If the insulating resin binder is properly chosen, a photosensitive layer of photoelectrograph containing titanium oxide as photoconductive fine powders can provide an electrostatic photographic material that can be charged both positively and negatively.

Unfortunately, a photosensitive layer containing titanium oxide has an extremely low sensitivity which is about l/lOOth of the sensitivity of a photosensitive layer comprising zinc oxide, and this has constituted a great barrier against the commercial availability of a photographic layer containing titanium oxide.

The use ofa mixture oftitanium oxide and zinc oxide can give a photosensitive layer having a higher sensitivity than that of a photosensitive layer containing titanium oxide alone and having the capability of being charged positively. However, in order to render the photosensitive layer positively chargeable, titanium oxide must be present in a fairly large amount, and because of this, the sensitivity of the photosensitive layer can be increased only to about one-third of that of a photosensitive layer containing zinc oxide alone.

A method of synthesizing novel photoconductive powders has now been found that a photoconductive layer that can be charged in both polarities can be prepared using these novel photoconductive powders.

Accordingly, a primary object of this invention is to provide an electrophotographic material that can be charged both positively and negatively.

Another object ofthis invention is to provide an electrophotographic material having a superior whiteness.

Still another object of this invention is to provide an electrophotographic material which after being charged and exposed imagewise, has a low residual potential on the surface of the photosensitive layer.

SUMMARY OF THE INVENTION It has now been found that the above objects of this invention can be achieved by using fine powders of a mixture of zinc oxide and an oxide oftitanium obtained by the hydrolysis of an organic compound of titanium.

DETAILED DESCRIPTION OF THE INVENTION The zinc oxide to be used in the present invention should preferably be one prepared by the vapor-phase oxidation method (French method) and having superior electrostatic photographic properties.

Examples of organic compounds of titanium used in the invention are tetrabutyl titanate, tetraisopropyl titanate, tetrahexyl titanate, tetrastearyl titanate, titanic acid esters of the general formula TitOR), wherein R is an alkyl or aryl group, or acyl titanates wherein one of the R groups is an acyl group. These organic compounds of titanium are easily hydrolyzed, and a very finely divided oxide of titanium can be obtained.

In order to achieve the objects of this invention effectively, the proportion of zinc oxide in the mixture should preferably be considerably larger than that of the oxide of titanium, and the mixing of these two powders should preferably be as uniform as possible.

Zinc oxide has especially good wettability with methanol and ethanol, and can be uniformly dispersed quite easily in such a liquid. Accordingly, in the process of this invention, the mixing of zinc oxide and the oxide of titanium is performed by uniformly dispersing zinc oxide in such an alcohol to make a sol, adding water in an amount equivalent to the organic compound of titanium to the sol. and then adding the organic compound of titanium gradually to the sol thereby to effect the hydrolysis of the organic compound of titanium. This makes it possible to distribute the oxide of titanium uniformly in a dispersion of zinc oxide in the alcohol.

The addition of the organic compound of titanium can be performed especially effectively by concurrently subjecting the zinc oxide dispersion to ultrasonic irradiation or mechanical stirring. The powders of zinc oxide and the oxide of titanium are separated from the liquid of the resultant mixture using procedures such as centrifugal separation. The powders are dried to form mixed powders. The reaction residues such an butanol or isopropanol cannot be completely removed unless the powders are dried at high temperatures for long periods of time, but can be completely removed by the heat-treatment step which follows. Experimental work showed that this difference in drying treatment barely affects the properties of the photographic material obtained.

The above described method is very effective for mixing zinc oxide and the oxide of titanium uniformly.

The objects of the present invention can be fully achieved also by an alternative mixing method which comprises preparing the oxide of titanium by the hydrolysis of an organic metal compound of titanium in a separate step, and mixing the oxide of titanium obtained with zinc oxide. Various methods of mixing are applicable to the present invention so long as they can effect uniform mixing of zinc oxide with the oxide of titanium obtained by the hydrolysis of an organic compound of titanium, before the heat treatment.

The mixed fine powders obtained can be heat-treated using, for example, an electric muffle furnace. The atmosphere in the furnace may preferably be an oxidizing atmosphere. Satisfactory results can be obtained with stationary air as an oxidizing atmosphere.

The mixed powders of zinc oxide and the titanium oxide so obtained are thoroughly kneaded with, and dispersed in, a resin varnish as the insulating resin binder to form a coating solution. The coating solution is coated on a suitable base material to produce an electrophotographic material suitable for the objects of this invention.

The mixing proportion between zinc oxide and the oxide oftitanium is such that from 3 to 7() mols, preferably from l5 to 40 mols, of zinc is present per mol of titanium. 0

Increasing amounts of zinc oxide give increasing sensitivity,vbut at the same time, present the difficulty in positive charging. The upper limit of the proportion of zinc oxide is determined based on consideration of this. On the other hand, when the amount of the oxide of titanium is larger, the resulting electrophotographic ma-- terial is easier to charge positively, but is decreased in sensitivity. Furthermore, when the heat-treatment temperature is high, a large amount of the oxide oftitanium leads to the formation ofa compound oxide of titanium and zinc. The compound oxide of titanium and zinc is photoconductive, but when an electrophotographic layer containing this compound oxide is charged and exposed image-wise, the layer has a high residual potential which makes it impossible to give a developed image of good quality. This is, of course, a disadvantage from a practical standpoint.

Known compound oxides of titanium and zinc include zinc ortho-titanate (Zn TiO,) having a ZnO-to- TiO mol ration of 2:1 zinc meta-titanate (ZnTiO having a ZnO-to-Ti0 mol ratio of l l and zinc titanate (Zi'lgTigOg) having aZnO-to-TiO; mol ratio of 2:3. Such a compound oxide is formed only with considerable difficulty when a mixture of conventional titanium oxide powders and zinc powders is used; it is formed by heat treatment at high temperatures for prolonged periods of time. However, powders of an oxide of titanium obtained from an organic metal compound of titanium have a much greater tendency to form a compound oxide with zinc oxide than conventional titanium oxide powders. Accordingly, special care must be taken with respect to mixing proportion of the oxide of titanium obtained by the hydrolysis of an organic metal compound of titanium. I

Experiments have shown that the upper limit of the mixing proportion of the oxide of titanium should be one mol, as titanium, per 3 mols of zinc. Proportions in excess of this upper limit result in the formation of a compound oxide of titanium and zinc which increases the residual potential of the resulting photosensitive layer and fails to give a feasible, electrophotographic material. The formation of the compound oxide can be confirmed using an X-ray diffraction analysis.

The resulting mixed powders are preferably heattreated at a temperature not lower than 300C. If the heat-treatment temperature is below 300C, the photosensitive layer is not charged with a positive polarity, and the negatively charged voltage is very low. In addition, the photosensitive layer has a low charge acceptance retention power, and the decay of the potential is rapid.

If the heat-treatment temperature is below 300C., therefore, the objects of this invention cannot be achieved, and it is impossible to produce a feasible, electrophotographic material capable of giving a high development density. With high heat-treatment temperatures, the positive chargeability of the photosensitive layer increases. Heat treatment at 300C will bring about satisfactory results for the objects of this invention, but preferably, temperatures of at least 500C. should be employed. It is desirable that the heattreatment should be carried out in an oxidizing atmosphere.

In most cases, a sufficient heat-treatinh time is from 2 to 3 hours or more although this varies somewhat depending upon the capacity of the heating equipment and the amount of the mixed powders of an oxide of titanium and zinc oxide.

There is no critical upper limit of the heat-treatment temperature, but no significant advantage is obtained by employing too high temperatures. The positive chargeability of the photosensitive layer is better with higher heat-treatment temperatures, but reaches a saturation point at temperatures in excess of 800C. if the temperature is further increased to more than 900C., a large agglomerated mass is formed as a result of the sintering reaction of the zinc oxide. Therefore, the mixed powders are not thoroughly dispersed in a binder resin, and a photosensitive layer having a smooth surface, cannot be obtained. Moreover, a residual potential is generated, or the powders turn yellow due to the defects of the oxygen atom in the crystal lattic of the zinc oxide. These phenomena tend to occur with a larger mixing proportion of zinc oXide and a longer heat-treatment time.

Accordingly, it is desirable that the heat-treatment temperature should not be higher than 900C. This is advantageous also from the standpoint of the cost of the powders obtained.

' The powders obtained in the above described conditions are fully kneaded with, and dispersed in, a resin varnish as a vehicle using a suitable kneader to form a coating solution. The coating solution is coated on a suitable base, such as paper or resin films, which has been treated to render it electrically conductive, and an electrophotographic material is thus produced.

The preparation of this electrostatic photographic coating solution can be effected under the same conditions as those used to prepare electrophotographic photosensitive layers generally used, and there is no particular restriction on the conditions of preparation.

The electrophotographic layer in accordance with this invention can be charged both positively and negatively. Therefore, even when an original is changed from a positive to a negative, the developer does not need to be changed, but it is only necessary to change the polarity of the layer charge. Consequently, the equipment in which the electrophotographic material of this invention can be used may be of a smaller size, and less maintenance is required.

Since the mixing proportion of titanium oxide in the photosensitive layer of this invention can be reduced to an extremely small extent, there is the advantage that the photographic layer can be charged both positively and negatively without decreasing the sensitivity of the photosensitive layer as much from that ofa photosensitive layer containing zinc oxide alone as the photoconductive powder.

In this prior art, a photosensitive layer comprising a mixture of zinc oxide and titanium oxide can also be charged both positively and negatively. However, titanium oxide must be incorporated in a high mixing proportion than in the case of the present invention, and this causes a drastic decrease in sensitivity.

Thus, the feature of this invention is that even if the mixing proportion of titanium dioxide is smaller than that of the prior art, a photosensitive layer chargeable both positively and negatively can be obtained.

An oxide oftitanium obtained by the hydrolysis of an organic metal compound of titanium is generally very fine and bulky, having a specific surface area of at least m /g. This oxide of titanium is amorphous. An

electrophotographic photosensitive layer obtained from it can barely be charged. When this oxide is heattreated, it is crystallized with a rise in the heattreatment temperature, and its crystal type changes from the anatase type to the rutile type. Thus, powders of titanium oxide for the production of the electrophotographic material can be obtained.

Since the powders ofthe titanium oxide used in this invention are extremely fine, it is assumed that although they are present in a considerably smaller pro portion than the zinc oxide, 'the titanium oxide powders are uniformly distributed in the mixed powders. Heattreatment improves the electrophotographic characteristics of the titanium oxide, whereby an electrophotographic material chargeable both positively and negatively can be provided. The advantages of this invention that the mixing proportion ofthe titanium oxide of titanium can be reduced may safely be ascribed to the fact that the titanium oxide obtained by the hydrolysis of an organic metal compound of titanium is extremely finely divided.

Titanium oxide has a higher degree of whiteness than zinc oxide. The whiteness of the mixed fine powders of zinc oxide and the oxide of titanium in the present invention is considerably higher than that of zinc oxide alone. When heat-treated, zinc oxide is used in admixture with the oxide of titanium in the present invention, the reduction in whiteness of the mixed powders is considerably smaller than the case of zinc oxide alone. This is another great advantage of this invention.

When the heat-treatment temperature exceeds 600C., a sintering ofthe zinc oxide begins, and the formation of large agglomerated masses is observed. This reduces the dispersibility of the powders, and a photosensitive layer with a smooth surface cannot be obtained. On the other hand, titanium oxide begins to be sintered at above 800C.

In spite of the fact that the mixed powders used in this invention contain a major proportion of zinc oxide, the heat-treatment of the powders at a temperature above 600C. does not give rise to the formation of large agglomerated masses, believed to be due to sintering, but the powders have good dispersibility. Thus, there can be obtained a photosensitive layer with a smooth surface, which has high utilitarian value.

When fine powders of titanium oxide produced by a conventional method such as the sulfuric acid method are mixed with zinc oxide in the same proportion as specified in the present invention, large agglomerated masses believed to be due to sintering are formed upon heat-treatment, and the same effect as in the present invention cannot be obtained.

This advantage of this invention that the dispersibility of the powders is not reduced even by heat-treatment at high temperatures may also be attributed to the fact that the oxide of titanium obtained by the hydrolysis of an organic metal compound of titanium is uniformly distributed in the mixed fine powders.

The mixed fine powders used in this invention can be characterized to some extent by an X-ray diffraction analysis.

When the mixing proportion of titanium oxide is low, substantially only the diffraction peak of zinc oxide appears as a result of a heat-treatment at a temperature below 600C. When the heat-treatment temperature exceeds 600C, the diffraction pattern of the rutile-type titanium oxide appears together with the diffraction peak of the zinc oxide. When the mixing proportion of the titanium oxide increases, the diffraction peak of the anatase-type titanium oxide is observed at a heattreatment temperature below 500C. When the temperature exceeds 500C, the anatase-type titanium oxide changes to the rutile-type titanium oxide, and the diffraction pattern of a compound oxide of zinc and titanium also appears. Within the range of the mixing proportion specified in this invention, the compound oxide is frequently zinc titanate (Zn Ti O,,) having a ZnO-to-TiO mol ratio of 2:3.

The sensitivity of the electrophotographic layer obtained in accordance with this invention can be increased by color sensitization using conventional techniques as disclosed in U.S. Pat. No. 3,052,540. Examples of color sensitizing dyes useful in this invention are triphenylmethane, phthalein or xanthene dyes such as Auramine, Fluorescein, Rose Bengale, Primoflavine, Malachite Green, Methylene Blue, Eosine, Erythrosine, Rhodamine B, Bromophenol Blue, Brilliant Blue FGF, or Phloxine.

The invention will now be illustrated in greater detail by reference to the following Examples and Comparative Examples. All parts and percents are by weight unless otherwise indicated.

EXAMPLES 1 TO 16 AND COMPARATIVE EXAMPLES 1 TO 14 100 parts by weight of fine powders of zinc oxide (tradename Sazex 2,000, product of Sakai Chemical Co. Ltd., produced by the vapor-phase oxidation method; average particle-size 0.6,u.) were dispersed in 400 parts by weight of methanol to form a uniform dispersion. The dispersing operation was performed using mechanical stirring and ultrasonic vibration. This dispersing operation gave a paste-like uniform zinc oxide/- methanol dispersion. More than an equivalent amount of water required for the subsequent hydrolysis had been added to the methanol.

A methanol solution of tetrabutyl titanate was added dropwise to this uniform dispersion and the tetrabutyl titanate was hydrolyzed to an oxide of titanium. The concentration oftetrabutyl titanate in the methanol solution was about 40 percent by weight. The hydrolysis was performed by applying mechanical stirring and ultrasonic vibration to the dispersion to which the methanol solution of tetrabutyl titanate was being added dropwise.

The mixed fine powders of zinc oxide and the titanium oxide were separated from the dispersing solvent using a centrifugal separator. The separated cake was placed in a dryer at 50C, and dried for one day to remove the methanol and residual butanol. The dried fine powders had an odor characteristic of butanol presumably because the butanol could not be removed completely. Even when water was added to the dispersing solvent separated by the centrifugal separator, powders were no longer formed. This indicates that the hydrolysis was completely performed.

The dried fine powders so obtained were placed in a porcelain crucible, and heat-treated in an electric muffle furnace. The rate of temperature increase in the heat-treatment was 25C per minute, and when the desired temperature was reached, the powders were maintained at that temperature for 2 to 3 hours to complete the heat-treatment.

Whether the heat-treated powders were withdrawn from the furnace immediately or after annealing, this essentially did not give rise to any difference in the properties of the electrophotographic photosensitive layer finally obtained.

Using the heat-treated powders, a photosensitive layer was produced in the following method.

condensate of trimethylol propane and tolylene diisocyanate in the form of a ethyl acetate solution) A styrcnatcd alkyd resin varnish having an acid value of less than 8. a specific gravity of 0.96 0.97 and a Gardncr'Holdt viscosity of "T W", available from Japan Reichhold Company.

These ingredients were kneaded and dispersed in a ball mill using n-butyl acetate as a diluent. The resultant dispersion was coated on a resin film having an aluminum layer vacuum-evaporated. in order to accelerate the curing reaction of the resin in the coating layer the coated film was maintained at 50C for 24 hours.

The characteristics of the electrophotographic material obtained were measured.

The photosensitive layer was subjected to dark applying by leaving at dark place for at least 2 days, and then subjected to a positive or negative corona charge. Then, the potential decay characteristics of the photosensitive layer were determined (dark decay characteristics). Then, another part of the sample was cut out. Light rays of different illuminance were produced by using the ND. filter with a tungsten lamp, and the charged electrophotographic material was successively exposed to these light rays, whereupon the light decay characteristics of the photosensitive material were determined. From the group of light decay curves so obtained, the percentage of residual potential was calculated as follows: (V, V )/(V, V X (percent) wherein V, is the potential of the sample after an exposure time of t seconds at illumination l,

V, is the potential of the sample before exposure,

V is the initial potential of the sample at the time of measuring its dark decay, and

V, is the potential of the sample after a I second dark decay.

The percentage of residual potential was plotted as the ordinate, and values corresponding to the logarithm (I X t) of the amount of exposure as the abscissa to obtain a characteristic curve of the potential of the photosensitive layer.

On the basis of the measured values obtained above, the following characteristic values were determined.

Initial potential: V (volts) Potential residual rate of dark decay: (V UV X 100 (percent) Sensitivity: log E the logarithm of the amount of exposure on abscissa corresponding to a potential residual rate of 35 percent on ordinate on the characteristic curve of potential measurement).

Residual voltage: V (volts) (the measurement potential that does not decay further even when exposed for long periods of time).

Table 1 shows the conditions for the production of the mixed fine powders in the above Examples and ysis. Table 3 shows the electrophotographic characteristics of each photosensitive layer.

Comparative Examples. Table 2 shows the structures of the mixed fine powders using an X-ray diffraction anal- Table l Powders Heabtrealing Temperatures Mixing (C) Ratio (Zn:Ti 200 300 500 700 900 mol ratio) 25:1 Compara- Compara- Compara- Compara- Comparalive live Exlive Exlive Exlive Ex- Example I] ample l ample 7 4 ample 9 ample 11 3:1 Compara- Example 1 Example 5 Example 9 Example 13 live Example 2 15:1 Compara- Example 2 Example 6 Example Example 14 live Ex- 10 ample 3 40:1 Compara Example 3 Example 7 Example Example 15 tive Ex- 1 l ample 4 70:1 Compara- Example 4 Example 8 Example Example 16 live Ex- 12 ample 5 80:1 Compara- Compara- Compara- Compara- Comparative live Extive Exlive Exlive Ex- Example 14 ample 6 ample 8 ample l ample 12 Table 2 Examples and Heat-treatment ZnzTi Mol Structure of the Mixed Comparative Temperature Ratio of Fine Powders using an Examples (C) the Mixed Xray Diffraction Powders Analysis Comparative 200 2.5:1 Zinc oxidelZnO) only Example l do. 2 do. 3:1 do. do. 3 do. 15:1 do. do. 4 do. 40:1 do. do. 5 do. 70:1 do. do. 6 do. 80:1 do. do. 7 300 2.511 do. Example 1 do. 321 do do. 2 do. 15:1 do. do. 3 do. 40:] do. do. 4 do. 70:1 do. Comparative do. Xllzl do.

Example 8 do. 9 500 2.5:1 Zinc oxide (ZnO) and rutile type titanium 1 oxide Example 5 do. 3:] do.

do. 6 do. 15:1 Zine oxide (ZnO) onlydo. 7 do. 40:1 do. do. 8 do. 70:] do. Comparative do. 8011 do.

Example 10 do. 1] 700 2.5:] Zinc oxide (ZnO), rutile type titanium oxide. and zinc titanate (Zn Ti O,.) lixample 9 do. 3:1 do. do. 11] do. 151'] do. do. 1 1 do. 40:1 Zine oxide (ZnO), and

zine titanale (Zn Ti Op do. 12 do. 70:1 Zinc oxide (ZnO) only Comparative 700 80:1 Zinc oxide (ZnO) only Example 12 do. 13 900 2.5:1 Zinc oxide (ZnO), rutile type titanium oxide. and zinc titanate (Zn h Example 13 do. 3:1 do. do. 14 do. l5:l do. do. 15 do. :1 Zinc oxide (ZnO). zine titanate lZn Ti O and zinc metatitanate (Z TiQ do. 16 do. :1 Zine oxide (ZnO) only Comparative do. :1 do.

Example 14 1 1 12 Table 3 Examples and Electrostatic Photographic Characteristics (umparative Initial Dark Decay Sensitivity Residual Examples Potential Residual Potential (V) Potential (V) Percentage (70) Comparative not Example 1 charged do. 2 do.

do. 3 do. do. 4 do.

do. 5 not positively charged 40 do. 6 not posilively charged 60 O do. 7 +8 0 -90 12 not measurable Example 1 +30 63 2.52 0 l 15 50 2.50 0 do. 2 +130 45 2.45 0 lO 52 2.45 0 do. 3 +60 60 2.35 0 +180 73 2.32 0 do. 4 +40 68 2.35 0 2l() 82 2.20 0 Comparative not posi- Example 8 tively charged 0 80 2.2] 0 do. 9 +53 33 3.54 12 l 60 3.22 0 Example 5 +80 40 2.73 (J l30- 63 2.52 0 Example 6 +200 62 2.48 (l 430 73 2.40 0 d0. 7 +280 83 2.4] 0 400 88 2.3] 0 do. 8 +80 73 2.l4 0 380 85 218 0 Comparative 87 not Example [0 measurable 380 87 2.l3 0 do. l l +3l 89 4.23 18 430 90 4.0l 0 Example 9 +250 87 3.86 less than l 450 92 3.42 do. 10 +280 87 3.43 0 -4l() 94 3.10 0 do. I l +l 90 2.67 0 405 92 2.48 0 d0. l2 63 2.55 0 420 90 2.31 0 Comparative +7 0 not Example l2 measurable 400 96 2.22 0 do. 13 +280 87 4.l6 l3 4l0 92 3.98 I 0 Example 13 +300 88 3.8l 4 -430 92 3.44 0 do. 14 +270 88 3.48 0 400 93 3.2] 0 do. 15 +l 9| 2.72 0 405 90 2.51 0 do. l6 +45 66 2.48 0 -4l0 93 2.25 0 Comparative +4 0 not Example l4 measurable EXAMPLE l7 initial Potential 2 v J When tetraisopropyl tl tanate was used as the organic Dark Decay Residual 22 ggr metal compound of titanium in Example 10 above, sub- Potential Percentage stantially the same results were obtained.

EXAMPLE l9 330 volts Initial Potential Dark Decay Residual Potential Percentage 230 volts 89% EXAMPLE is When tetrastearyl titanate was used as the organic metal compound of titanium in Example 10, substantially the same results were obtained.

When 0.3 part by weight each of Fluorescein and Rhodamine B was added as a sensitizing dye in Example l0, considerable increase in sensitivity as shown by the following results was obtained.

Initial Potential 250 volts 380 volts Dark Decay Residual 88% 9371 Potential Percentage Sensitivity (log E 2.56 2.27

EXAMPLE lnitial Potential 330 volts 350 volts Dark Decay Residual 82% 83?! Potential Percentage Sensitivity 2.45 2A7 It can be seen that the difference between the positive charging properties and the negative charging properties is small.

EXAMPLE 21 In the procedure of each of Examples 1 to 16, fine powders of an oxide oftitanium were separately prepared by hydrolyzing tetrabutyl titanate, and then they were mixed with zinc oxide. The mixture was heattreated to form mixed fine powders of titanium oxide and zinc oxide.

As far as the X-ray diffraction analysis indicated, a compound oxide of titanium and zinc seemed to be more difficultly formed than in each of Examples 1 to 16. There was hardly any difference in the properties of the photosensitive layers obtained.

While the invention has been described in detail and in terms of specific embodiments thereof, it will be apparent that various changes and modifications can be made therein without departing from the spirit and scope thereof.

What is claimed is:

l. A method for the preparation of an electrophotographic material that can be charged both positively and negatively, which electrophotographic material comprises a powdery mixture of zinc oxide and an oxide of titanium in a molar ratio of 3:l to :l based on the zinc and the titanium, comprising: obtaining said powdery mixture by adding an organic compound of titanium to a zinc oxide-sol containing water to effect the hydrolysis of the organic compound oftitanium and heat-treating said powder mixture at a temperature of 300900C.

2. The method of claim 1. further comprising tiniformly distributing said powdery mixture in an electrically. insulating resin binder and coating the resulting material on an electrically conductive base.

3. The method of claim 1, wherein said zinc oxide-sol is made from zinc-oxide and an alcohol.

4. The method of claim 3, wherein said alcohol is methanol or ethanol.

5. The method of claim 1, wherein the amount of said water is equivalent to the organic compound of titanium.

6. The method of claim 1, wherein said zinc oxide is obtained by the vapor phase oxidation method.

7. The method of claim 1, wherein said organic compound of titanium is a titanic acid ester or an acyl titanate.

8. The method ofclaim 7, wherein said organic compound of titanium is tetrabutyl titanate, tetraisopropyl tit-anate or tetrastearyl titanate.

9. The method of claim 1, wherein said heat-treating is in an oxidizing atmosphere.

10. The method of claim 1, wherein said heattreating is for from 2-3 hours. 

2. The method of claim 1, further comprising uniformly distributing said powdery mixture in an electrically insulating resin binder and coating the resulting material on an electrically conductive base.
 3. The method of claim 1, wherein said zinc oxide-sol is made from zinc-oxide and an alcohol.
 4. The method of claim 3, wherein said alcohol is methanol or ethanol.
 5. The method of claim 1, wherein the amount of said water is equivalent to the organic compound of titanium.
 6. The method of claim 1, wherein said zinc oxide is obtained by the vapor phase oxidation method.
 7. The method of claim 1, wherein said organic compound of titanium is a titanic acid ester or an acyl titanate.
 8. The method of claim 7, wherein said organic compound of titanium is tetrabutyl titanate, tetraisopropyl titanate or tetrastearyl titanate.
 9. The method of claim 1, wherein said heat-treating is in an oxidizing atmosphere.
 10. The method of claim 1, wherein said heat-treating is for from 2-3 hours. 